Modular gas injection device

ABSTRACT

Embodiments of the device relate to a modular injector ( 100 ) for injecting a gas into a processing chamber ( 42 ), comprising at least two adjacent injectors ( 1 ), each injector comprising an inlet for receiving a gas wave or a gas flow, a flow shaping section ( 2 ) having left and right sidewalls that diverge according to a divergence angle relative to a propagation axis of the gas, for expanding the gas in a direction perpendicular to the propagation axis, and an outlet for expelling the gas. The modular injector forms an equivalent large injector having an equivalent large outlet which includes the outlets of the adjacent injectors and expands the gas over the equivalent large outlet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Section 371 of International Application No.PCT/IB2011/051274, filed Mar. 25, 2011, which was published in theEnglish language on Oct. 6, 2011, under International Publication No. WO2011/121508 A1 and the disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a gas injection device and to a methodfor injecting a gas into a processing chamber. The present inventionrelates in particular to a method for injecting a gas into a processingchamber of a thin film reactor.

Thin film deposition techniques, such as Physical Vapor Deposition (PVD)and Chemical Vapor Deposition (CVD), are techniques for depositing thinfilm layers upon a substrate, such as a semiconductor substrate. Oneparticular CVD process subclass, called Atomic Layer Deposition ALD(also known as Atomic Layer Epitaxy ALE or Atomic Layer Chemical VaporDeposition ALCVD), is used for semiconductor and thin film magnetic headmanufacturing, and is being considered for the manufacturing of variousnew devices such as organic light emitting displays (OLED's) andphotovoltaic elements. FIG. 1A shows a conventional thin film depositionsystem TFS1 comprising at least one injector 1 with a flow-shapingsection 2, a processing chamber 4 wherein a substrate 5 may be placed,and an exhaust device 6. A gas tube 7 links the injector 1 to at leastone source gas 8.

During a deposition process, a carrier gas comprising reactants isgenerally introduced into the injector 1 during a certain period oftime, thereby forming a “gas wave” or “gas pulse”. As described in U.S.Pat. No. 7,163,587, the flow-shaping section 2 of the injector may havea triangular shape with first and second sidewalls diverging accordingto a constant divergence angle relative to a propagation axis XX′ of thegas wave inside the injector. The flow-shaping section 2 laterallyexpands the gas wave as it travels from a point O at the inlet of theflow-shaping section 2 until it reaches an outlet of the injector thatopens onto the processing chamber 4. The gas is then expelled into theprocessing chamber 4, as shown by arrows in FIG. 1A, wherein reactantsin the gas may react with the substrate surface 5 and/or withpreviously-deposited molecules. The processing chamber 4 may then bepurged by injecting an inert gas that clears any excess reactants andproducts from the system, which are evacuated by means of the exhaustdevice 6. The pulsing/purging steps may then be repeated with a secondgas from another gas source. Thin layers, for example between 0.1 and 3Å, may be formed upon the substrate 5. This cycle is repeated as manytimes as necessary to obtain the desired thin film thickness.

Due to its layer-by-layer implementation, Atomic Layer Deposition allowsfor very high structural quality and thickness control of the thin filmlayers, as well as good step coverage over any features that may bepresent on the substrate. However, due to the required pulsing andpurging steps, this process may take anywhere from several minutes toseveral hours, depending upon many factors such as the desired thin filmthickness, reactants used, rapidity of the cycling, etc., resulting in arelatively low throughput. Recent research and development has focusedon decreasing the deposition time of thin films in order to make thistechnique more attractive for large-scale production.

One common way to decrease the cycle time is to increase the gas flowrate. However, due to the Poiseuille effect, the triangular shape of theflow-shaping section causes the gas in the center of the injector toarrive at the outlet before the gas near the sidewalls. Thus, when suchan injector is used in an application where a time-sequenced compositionprofile is created at the entrance, this peaked velocity distributionwill result in a non-uniform gas composition distribution. Such anon-uniform gas distribution slows down the process by increasing theamount of time for a gas wave to travel through the processing chamber,as will be explained in relation with FIGS. 1B, 1C.

FIG. 1B shows the profile C01 of the gas velocity V at the outlet of theinjector 1 of FIG. 1A. The gas velocity is measured along an axis YY′perpendicular to the gas propagation axis XX′ and is expressed in metersper second. It can be seen that the gas velocity profile C01 has a peakvalue at a point O′ at the center of the outlet of the injector, andquickly decreases when going away from point O′, to reach 0 at thevicinity of the sidewalls of the injector. FIG. 1C shows two profilesP01, P02 of gas concentration GC along the propagation axis XX′ throughthe processing chamber 4, from point O′ at the outlet of the injector,at two different times after the injection of the gas wave. The gasconcentration is expressed in percentage of the reactant present in thegas per unit of volume. It can be seen that, due to the diffusion of thereactant in the carrier gas, the length of the gas wave or gas pulseincreases as it travels through the processing chamber.

In view of FIGS. 1B, 1C, it can be understood that a relation existsbetween the length of the gas wave P01 and the gas velocity profile C01,such that the more non-uniform the gas velocity profile along YY′ at theoutlet of the injector, the longer the time to expel the gas from theinjector, the longer the profile of the gas wave along XX′ that travelsthrough the processing chamber, and the longer the time required betweenthe injection of two successive gas pulses, since the slowest part ofthe gas wave must exit the chamber before the next wave is injected.

A non-uniform gas distribution may also cause deposition anomalies anduneven thin films. Therefore, it may be desirable to have as uniform adistribution of the gas across the surface of the substrate as possible.The gas wave should be optimized for the thin film deposition system andis highly dependent upon the physical properties of the injectors,dimensions of the processing chamber, substrate to be deposited upon,etc. As thin film deposition systems are typically used for electronicapplications, they are therefore optimized for the standardizedsubstrate dimensions often used in this industry, such as 150, 200, and300 mm diameters. In order to apply these techniques to larger substratesizes for other application domains, the dimensions of the injectors,gas inlets, processing chamber, exhaust device, etc. must be modifiedaccordingly, complicating and increasing the cost of thin filmdeposition systems for large (500 mm or more) substrate sizes with fastcycling times. It is neither practical nor economical to develop asystem for each possible processing combination, and current systems arenot well-adapted for larger substrates, limiting their utility for otherdomains of application.

Furthermore, recent interest has arisen for the application of thin filmdeposition methods to other industries such as glass coating, display,and photovoltaics. These applications use much larger substrates, suchas 1200 by 600 mm glass plates or a continuous roll of flexiblematerial, requiring an increase in the amount of gas traveling throughthe injector, and a widening of the outlet of the injector. However,increasing the gas flow rate and widening the outlet of the injectorcause turbulence and recirculation of the gas in the injector, resultingin an even more non-uniform distribution of the gas across the substratesurface and inefficient purging of precursors from the system.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention relate to a modular injector forinjecting a gas into a processing chamber, characterized in that itcomprises at least two adjacent injectors, each injector comprising: aninlet for receiving a gas wave or a gas flow; a flow shaping sectionhaving left and right sidewalls that diverge according to a divergenceangle relative to a propagation axis of the gas, for expanding the gasin a direction perpendicular to the propagation axis; and an outlet forexpelling the gas. The modular injector forms a substantially equivalentlarge injector having an equivalent large outlet including the outletsof the adjacent injectors and expanding the gas over the equivalentlarge outlet.

According to one embodiment, the modular injector comprises a connectionarea extending between adjacent sidewalls of the injectors, wherein eachinjector comprises means for expelling the gas in the vicinity of theconnection area with a greater flow rate than near the center of itsoutlet, to compensate for the lack of gas expulsion in the connectionarea.

According to one embodiment, the injectors are configured so that thegas expelled by the modular injector through the equivalent large outlethas a velocity profile showing a variation of less than 10% betweenmaximum and minimum velocities over at least 90% of the width of theequivalent large outlet, at a certain distance from the outlet and theconnection area.

According to one embodiment, each injector comprises a diffuser platecomprising a plurality of openings for the passage of the gas, theopenings being sized and/or spaced from each other so that the injectorexpels the gas in the vicinity of the connection area with a greaterflow rate than near the center of its outlet.

According to one embodiment, the flow-shaping section of each injectorcomprises at least a constriction region where a height of theflow-shaping section varies along an axis perpendicular to thepropagation axis and presents a first height near the center of theflow-shaping section and a second height near the sidewall close to theconnection area, the first height being less than the second height toslow down the velocity of the gas near the center of the gas relative tothe velocity of the gas near the sidewall close to the connection area.

According to one embodiment, the flow-shaping section of each injectorhas a first expansion region where the sidewalls diverge according to afirst divergence angle, and a second expansion region including theconstriction region, where the sidewalls diverge according to a seconddivergence angle smaller than the first divergence angle, to acceleratethe velocity of the gas near the sidewalls close to the connection arearelative to the velocity of the gas near the center of the flow-shapingsection.

According to one embodiment, the first divergence angle varies andpresents a largest value at the end of the first region, and the seconddivergence angle is constant and smaller than the largest value of thefirst divergence angle.

According to one embodiment, the modular injector has a curved flowshaping section.

Embodiments of the invention also relate to a system comprising: aprocessing chamber, at least one modular injector according to theinvention, the equivalent large outlet of which opens into theprocessing chamber, and at least one gas source coupled to the inputs ofthe injectors of the modular injector.

According to one embodiment, the system comprises at least twosuperposed modular injectors arranged so that each injector of eachmodular injector has a common outlet with one injector of the othermodular injector.

Embodiments of the invention also relate to a method for injecting a gaswave or a gas flow in a processing chamber, comprising: expanding thegas in a direction perpendicular to a propagation axis of the gas; theninjecting the gas into the processing chamber. The method furthercomprises injecting the gas in the processing chamber with a modularinjector comprising at least two adjacent injectors, each injectorcomprising an inlet for receiving a gas, a flow-shaping section havingleft and right sidewalls which diverge according to a divergence anglerelative to a propagation axis of the gas, for expanding the gas in adirection perpendicular to the propagation axis, and an outlet forexpelling the gas, the modular injector forming an equivalent largeinjector having an equivalent large outlet including the outlets of theadjacent injectors and expanding the gas over the equivalent largeoutlet.

According to one embodiment, the modular injector comprises a connectionarea extending between adjacent sidewalls of the injectors, and themethod comprises configuring each injector so that it expels the gas inthe vicinity of the connection area with a greater flow rate than nearthe center of its outlet, to compensate for the lack of gas expulsion inthe connection area.

According to one embodiment, the method comprises configuring eachinjector so that the gas expelled by the modular injector has a velocityprofile showing a variation of less than 10% between maximum and minimumvelocities over at least 90% of the width of the equivalent largeoutlet.

According to one embodiment, the method comprises providing, in eachinjector, a diffuser plate having a plurality of openings for thepassage of the gas, the openings being sized and/or spaced from eachother so that the injector expels the gas in the vicinity of theconnection area with a greater flow rate than near the center of itsoutlet.

According to one embodiment, the method comprises providing, in theflow-shaping section of each injector, at least a constriction regionwhere a height of the flow-shaping section varies along an axisperpendicular to the propagation axis of the gas and presents a firstheight near the center of the flow-shaping section and a second heightnear the sidewall of the flow-shaping section close to the connectionarea, the first height being less than the second height to slow downthe velocity of the gas near the center of the injector relative to thevelocity of the gas near the sidewall close to the connection area.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown. In thedrawings:

FIG. 1A, previously described, is a schematic top view of a conventionalthin film deposition system;

FIG. 1B, previously described, shows a velocity profile of a gastraveling through the system of FIG. 1A;

FIG. 1C, previously described, shows a concentration profile of a gastraveling through the system of FIG. 1A;

FIGS. 2A and 2B are schematic side and top views of a thin filmdeposition system according to a first aspect of the invention;

FIG. 3 is a top view of a first embodiment of an injector of the systemof FIGS. 2A, 2B;

FIGS. 4A, 4B, 4C, 4D are cross-sectional views of the injector of FIG. 3along different cross-sectional planes;

FIGS. 5A, 5B, 5C, 5D show different velocity profiles of a gas travelingthrough the injector of FIG. 3, at different points of the injector;

FIG. 6 shows variations of a velocity profile of a gas at an outlet ofthe injector of FIG. 3, for different inlet velocities of the gas;

FIG. 7 shows different concentration profiles of a gas traveling throughthe system of FIGS. 2A, 2B;

FIG. 8 is a cross-sectional view of an injector assembly of the systemof FIGS. 2A, 2B;

FIG. 9 is a perspective view of a second embodiment of an injector ofthe system of FIGS. 2A, 2B;

FIG. 10 is a schematic top view of a first embodiment of a thin filmdeposition system according to a second aspect of the invention;

FIG. 11 is a schematic top view of a second embodiment of a thin filmdeposition system according to the second aspect of the invention;

FIGS. 12A and 12B show diffuser plates present in the system of FIG. 11;

FIG. 13 is a schematic top view of a third embodiment of a thin filmdeposition system according to the second aspect of the invention;

FIG. 14 shows the velocity profile of a gas traveling through the systemof FIG. 13; and

FIGS. 15A, 15B, 15C show different gas circuit configurations of a thinfilm deposition system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of a thin film deposition system TFS2 according to a firstaspect of the invention is schematically shown in FIG. 2A (side view)and FIG. 2B (top view). For the sake of clarity of the drawings, theelements are not necessarily shown to scale and the drawings do notnecessarily show every aspect of the system.

The system TFS2 is designed for Atomic Layer Deposition applications andcomprises an injector assembly 10 having two injectors 11, 11′, a baseunit 40, and an exhaust device 61. The base unit 40 comprises aprocessing chamber 41 receiving a substrate 50, which may be mounted ona support. Each injector 11, 11′ has a gas admission inlet linked to agas source 81, 82 through a gas tube 71, 72 and a valve 85, 86. The baseunit 40 may be made out of metal, such as aluminum or stainless steel,and may comprise further components such as to create a vacuum, heat theprocessing chamber and/or the substrate, openings to allow theintroduction and removal of the substrate, cleaning, alignment of thebase unit with other components, etc.

A first gas wave is introduced into the injector 11 and propagates alonga propagation axis XX′ through the injector 11, wherein it is expandedlaterally, i.e. along an axis Y4Y4′ perpendicular to the propagationaxis XX′ before being expelled into the processing chamber 41. In theprocessing chamber, the first gas wave travels across the surface of thesubstrate 50 in a wave that is substantially parallel to the surface ofthe substrate, and reacts with the substrate before being purged fromthe chamber by means of the exhaust device, which is linked to a pump60.

A second gas wave is then introduced into the injector 11′ andpropagates along the propagation axis XX′ through the injector 11′,wherein it is laterally expanded along the axis Y4Y4′ before beingexpelled into the processing chamber 41. In the processing chamber, thesecond gas wave travels across the surface of the substrate 50 in a wavethat is substantially parallel to the surface of the substrate, andreacts with deposits left by the first gas injection.

As an example, to obtain an Al₂O₃ film a first gas comprising aluminum,such as trimethyl aluminum (TMA) or aluminum chloride (AlCl₃), is pulsedinto the processing chamber through the first injector where it reactswith the substrate, then the first gas is purged from the chamber bymeans of the exhaust device while injecting an inert gas, such asnitrogen N₂ or argon Ar, in the chamber through the first injector. Thena second gas comprising oxygen, such as water vapor H₂O or ozone O₃, ispulsed in the chamber through the second injector, and the oxygen reactswith the aluminum, forming a monolayer of Al₂O₃ film. The chamber isthen purged again with an inert gas, and the cycle is repeated.

FIG. 3 is a top view showing in further detail the structure of theinjector 11 according to a first embodiment of the invention. It isassumed in this embodiment that both injectors 11, 11′ have the samestructure, therefore only injector 11 will be described. However, otherembodiments may be provided in which injectors 11, 11′ have differentstructures, for example depending on the nature of the gas they areintended to inject.

The injector 11 comprises a body 12 and a flow-shaping section 20. Theflow-shaping section 20 may be a cavity milled out of the body 12, whichmay be for example a metal plate, such as aluminum or stainless steel.The cavity may be formed by precise milling of the metal plate. A coverdevice 13 (not shown in FIG. 3) may be attached to the body 12 bywelding, brazing, or simply mounted thereupon with screws, pins, and thelike.

The flow-shaping section 20 comprises an inlet 21 and an outlet 22. Theinlet 21 is connected to the gas tube 71 and has the shape of a smallopening. The outlet 22 has the shape of a larger opening and opens ontothe processing chamber, for expelling the gas thereunto. Theflow-shaping section comprises right and left sidewalls 23, a bottomsurface, and a top surface formed for example by the above-mentionedcover device. The bottom and top surfaces are substantially parallelexcept in a region described below. The sidewalls 23 diverge outwardlyfrom the inlet 21 to the outlet 22, expanding the gas wave in adirection perpendicular to a propagation axis XX′ of the gas wave.

The flow-shaping section 20 further comprises a constriction region 24to shape the gas velocity profile as desired. As will be explained infurther detail below, the height of the flow-shaping section 20 in theconstriction region 24 varies so as to standardize the velocity of thegas wave across the lateral expansion width of the gas wave expelledinto the processing chamber 41. The injector 11 therefore expands thegas wave along axes Y1Y1′, Y2Y2′, Y3Y3′, Y4Y4′ perpendicular to thepropagation axis XX′, while adjusting the velocity of the gas across thelateral expansion width. In particular, and as shown in FIG. 4C, theconstriction region may comprise a reduced height h2 in the vicinity ofthe propagation axis and a greater height h3 near the sidewalls 23, toslow down the velocity of the gas near the center of the flow-shapingsection relative to its velocity near the sidewalls.

In the embodiment shown in FIG. 3, the flow-shaping section furthercomprises a first expansion region 25 that performs only lateralexpansion and a second expansion region 26 that includes theconstriction region 24 and performs lateral expansion and verticalconstriction (regions 25, 26 are shown by different shadings).

The first expansion region 25 extends from the inlet 21 to a transitionpoint X2, and the second expansion region 26 extends from point X2 tothe outlet 22. The first and second expansion regions may be twodifferent pieces joined together, or may simply be different regions ofa same piece.

The first expansion region 25 comprises left sidewall 23 a and rightsidewall 23 b, and the second expansion region 26 comprises leftsidewall 23 c and right sidewall 23 d. The sidewalls 23 a, 23 b of thefirst expansion region 25 diverge according to a divergence angle A1relative to the propagation axis XX′ of the gas wave, whereas thesidewalls 23 c, 23 d of the second expansion region 26 diverge accordingto a divergence angle A2 relative to the propagation axis XX′. In oneembodiment, the divergence angle A1 is greater than the divergence angleA2, so that the second expansion region 26, all while performing anoverall lateral expansion of the gas wave, also performs an additionalconstrictor function near the sidewalls, to further decrease thevelocity of the gas near the center relative to its velocity near thesidewalls of the second expansion region 26.

In one embodiment, the divergence angle A1 varies and increases as thedistance from the inlet 21 increases, to reach a maximum value A1max atthe end of the first expansion region 25, while the divergence angle A2is constant, A1max being greater than A2. Preferably, the divergenceangle A1 varies according a supralinear function, such as a quadratic orexponential function. A supralinear lateral expansion helps to suppressturbulence near the inlet 21 of the injector 11, where the velocity ofthe gas is the highest. In the embodiment shown in FIG. 3, the sidewalls23 a, 23 b of the first expansion region 25 diverge exponentiallywhereas the sidewalls 23 c, 23 d of the second expansion region 26diverge linearly with the constant angle A2.

FIGS. 4A, 4B, 4C, 4D are cross-sectional views of the injector of FIG. 3along different cross-sectional planes, respectively P1, P2, P3, P4,which are perpendicular to the propagation axis XX′. Plane P1 passesthough a point X1 of the axis XX′, point X1 being at a distance d1 froma reference point O at the inlet 21. Plane P2 passes though a point X2of the axis XX′, point X2 at a distance d2 from point O, d2 beinggreater than d1 and equal to the length of the first expansion region25. Plane P3 passes though a point X3 of the axis XX′, point X3 at adistance d3 from point O, d3 being greater than d2 and correspondingsubstantially to the midpoint of the constriction region 24. Plane P4passes though a point X4 on the axis XX′, point X4 being at a distanced4 from point O, d4 being greater than d3 and equal to the length of theflow-shaping section 20, so that plane P4 includes the outlet 22 of theinjector. In FIG. 3, the propagation axis XX′ and a longitudinal axis ofsymmetry of the injector are one and the same, so that points X1, X2,X3, X4 are each equidistant from the right and left sidewalls of theflow-shaping section 20.

In FIG. 4A, the cross-section of the first expansion region 25 of theflow-shaping section 20 has a substantially rectangular shape, with awidth W1 and a height h1. The height h1 is the distance between thebottom surface and the top surface of the flow-shaping section 20, thetop surface being formed here by a lower face of the cover 13.

In FIG. 4B, the cross-section of the flow-shaping section 20, at thelimit between first and second expansion regions 25, 26, has asubstantially rectangular shape with a width W2>W1 and a height h1 equalto that of FIG. 4A.

In FIG. 4C, the cross-section of the flow-shaping section 20, in theconstriction region 24 located within the second expansion region 26,has a width W3>W2, a flat top surface and a substantially convex bottomsurface. Therefore, it presents a first height h2 in the middle ofregion 26 (i.e. near point X3) and a second height h3 near thesidewalls, h3 being here greater than h2 so as to reduce the gasvelocity in the center of the second expansion region relative to thegas velocity near the sidewalls. In one embodiment, the shape of theconvex bottom surface along axis Y3Y3′ is defined according to a Beziercurve. In this embodiment, h2 is greater than h1 but may be equal to orless than to h1 in other embodiments.

In FIG. 4D, the cross-section of the flow-shaping section 20, at thevicinity of the outlet 22, has a substantially rectangular shape with awidth W4>W3 and a height h4 less than h3 and also less than h1. In otherembodiments, h4 may be greater than h1 and less than h2.

In one embodiment, the bottom surface of the second expansion region 26comprises two non-uniform rational basis spline surfaces (NURBS), one inthe area between axes Y2Y2′ and Y3Y3′, and another in the area betweenaxes Y3Y3′ and Y4Y4′.

FIGS. 5A, 5B, 5C, 5D show different profiles C1, C2, C3, C4 of thevelocity V of a gas wave traveling through the injector, measured alongdifferent axes Y1Y1′, Y2Y2′, Y3Y3′, Y4Y4′ perpendicular to thepropagation axis XX′ and respectively included in planes P1, P2, P3, P4.A Computational Fluid Dynamics (CFD) analysis on a three-dimensionaldiscretized model of the injector geometry using the finite volumemethod may be used to obtain such profiles.

It can be seen that:

In FIG. 5A, profile C1 is strongly peaked around the central point X1 ofthe flow-shaping section 20 and shows a high velocity (for example 17m/s) at this point, then quickly decreases when going away from point X1to reach a velocity of 0 m/s at the vicinity of the sidewalls 23 (thevelocity of a gas traveling though a pipe being always being equal to 0right next to the walls of the pipe);

In FIG. 5B, profile C2 is less but still peaked around central point X2,shows a lower velocity (for example 5 m/s) at point X2, then decreasesless quickly than profile C1 when going away from the central point X2,to reach a velocity of 0 m/s at the vicinity of the sidewalls 23;

In FIG. 5C, it can be seen that the correction of the velocitydispersion between the center of the injector and the sidewalls hasbegun. Profile C3 is no longer peaked around the central point X3, andpresents a rather flat profile with abrupt slopes in the vicinity of thesidewalls 23. The larger height h3 causes reduced flow resistance nearthe sidewalls, guiding the gas towards the edges. As a result, the gashas a higher velocity, about 10%, near the edges than in the center.Thus, profile C3 shows a slight “dip” in the vicinity of the centralpoint S3 and two “bumps” on either side of the dip;

In FIG. 5D, profile C4 is substantially uniform across the majority ofthe outlet 22 and decreases abruptly to a velocity of 0 m/s in thevicinity of the sidewalls 23.

Therefore, the different heights h1, h2, h3, h4 of the injector can beadjusted to tune as desired the velocity profile of the gas expelledinto the processing chamber.

In applications where a uniform velocity profile is sought at the outletof the injector, the present invention allows a velocity dispersion lessthan 10% between maximum and minimum velocities over at least 90% of thewidth of the outlet to be obtained. Generally speaking, the velocitydispersion at the outlet 22 of the injector depends not only on thedifference between h1, h2, h3, h4, but also on the difference betweenthe maximum divergence angle A1 of the first expansion region 25 anddivergence angle A2 of the second expansion region 26, and on theinitial velocity of the gas at the inlet 21, as will now be seen inreference to FIG. 6.

FIG. 6 shows the effect of the inlet gas speed upon gas velocity profileon outlet of the injector 11, along axis Y4Y4′. The gas velocity isexpressed here as a ratio between the velocity of the gas along axisY4Y4′ and its maximum velocity. In this example, the first expansionregion 25 has a width W2 of 10 cm, a length d2 of 7.5 cm, and adivergence angle A1 varying exponentially. The constriction region 26has a height h3=0.5*h2. Three profiles C4′, C4″, C4′″ are shown,corresponding to inlet velocities at the inlet 21 of 10 m/s, 40 m/s, 70m/s respectively. Profiles C4′ and C4″ have two lateral “bumps” and onecentral “dip”, and therefore show one lowest velocity point near thecentral point X4 and two highest points near the sidewalls. Profile C4′″has three “bumps” and two “dips”, and has therefore one central maximalvelocity point and two lateral maximal velocity points, and two lowestvelocity points between the maximal velocity points. It is to be notedthat these velocity profile features are shown in an expanded view andare, in practice, relatively small. For example, for the profile C4′″,the difference between the “bump” and the “dip” represents a variationof less than 1% of the velocity of the gas, which is thereforesubstantially uniform over most of the width of the outlet of theinjector.

These profiles exhibit that for a determined shape of the expansionsregions 25, 26 and constriction region 24, a critical velocity VCexists. The injector has a different behavior depending on whether theinlet gas velocity (velocity of the gas at the inlet of the injector) isbelow or above the critical velocity VC. Below the critical velocity,there is no turbulence and recirculation in the first expansion region25; the velocity profile is substantially constant and independent ofthe inlet velocity. Above the critical velocity, a recirculation patternoccurs in the first expansion region 25, and the outlet velocity profilechanges with the inlet velocity, causing changes in the uniformity ofthe velocity profile. The velocity VC value depends on the geometry: ashort and/or wide first expansion region 25 has a lower VC, which meansthat recirculation or turbulence occur at a lower input velocitycompared to a longer and/or narrower expansion region. Therefore, thereexists an approximate scaling relationship of the type VC=f(d2/W2).

The outlet velocity profile can be tuned by adjusting the height h2 ofthe constrictor at point X2. The required correction depends on thefirst expansion region 25 length and the desired operating point. Amethod of designing the injector may consist in slightlyovercompensating the velocity near the sidewalls so that it can have itsmost uniform velocity profile at the highest allowable inlet velocitybefore the critical velocity is reached.

Generally speaking, it is within the capabilities of the skilled personto adjust the properties of the injector, for example by adjusting thedifferent heights h1, h2 h3, h4 in order to obtain the desired gasprofile in relation with a considered application. The height h1 may bechosen to approximately match the diameter of the inlet gas tube, whichmay have a standardized diameter of approximately 6 mm or approximately12 mm in some applications. If a height h1 value is chosen that issignificantly different from the inlet diameter, a step in the flow pathmay be created, causing recirculation or turbulence. The height h4 maybe relatively small, such as 1-2 mm in some applications, and may belimited by the manufacturing process. A small height h4 value may helpto prevent a “backflow” of gas from another injector if there is morethan one injector.

FIG. 7 shows profiles P10, P11, P12, P13 of gas concentration GC alongthe propagation axis XX′ through the processing chamber 41, from pointX4 at the outlet of the injector (Cf. FIG. 3), at four different timesafter the injection of the gas wave into the injector, respectively 80ms, 100 ms, 120 ms, and 140 ms. The gas concentration is expressed inpercentage of the reactant present in the gas per unit of volume (forexample TMA or oxygen). As the gas wave travels through the processingchamber it remains very uniform, without losing much of its height orbecoming very dispersed. This is due to the initial uniformity of thegas velocity profile at the outlet of the injector. The pulse width ofthe traveling wave increases slowly, with a small difference between thevalues near the sidewalls as compared to the values near the center ofthe injector. It has been observed that the thin film deposition systemis ready to begin to the next cycle after about half of the processingchamber has been purged, contrary to a conventional system such as thatdescribed in U.S. Pat. No. 7,404,984, in which a purge pulse of at leasttwice of the volume of the processing chamber must have been injectedbefore starting the next cycle. As a result, the monitoring of thevelocity profile, thanks to the structure of injector according to theinvention, allows the rate of the pulsing and purging cycles to beincreased.

FIG. 8 is a cross-sectional view of an embodiment of the thin filmdeposition system TFS2 of FIGS. 2A, 2B, showing how two injectors 11,11′ can be assembled to form the injector assembly 10. As previouslydescribed, each injector 11, 11′ comprises a machined body 12 in whichthe different regions 24, 25, 26 of the flow-shaping section 20 havebeen formed, and a cover 13. A divider block 87 is placed between theinjectors 11, 11′ for physical support. Each injector is arranged on thedivider block 87 so that its cover 13 is against the divider block. Seenin cross-section, the divider block has a triangular shape so that theinjectors 11, 11′ are oriented with respect to each other with a certainangle, for example about 60°, and so that their outlets 22 join togetherto form a common outlet. The common outlet is formed by an area wheretheir respective bodies 12 are not covered by the cover 13 and areopposite one another. The injectors 11, 11′ are attached to and alignedwith the base unit 40 by means of alignment devices 88, 89, such aspins. Each injector 11, 11′ is connected on inlet to one gas tube 71, 72respectively, linked to one or more gas sources (not shown).

In an alternative embodiment (not shown), the injectors may not comprisecover devices. The divider block 87 itself may form the top surfaces ofeach injector.

FIG. 9 shows an alternative embodiment of an injector 111 according tothe invention. The injector 111 comprises an inlet 112, followed by afirst flow-shaping section 113, itself followed by a second flow-shapingsection 114, and an outlet 115 at the end of the second flow-shapingsection 114. The first flow-shaping section 113 has a curved shape,approximately in an “S” shape. It also has sidewalls 116 which divergeaccording to a first divergence angle A1, preferably a variable angleincreasing according to a quadratic or exponential function. Therefore,the first flow-shaping section 113 forms an expander devicecorresponding to the first expansion region previously described. Apropagation axis X1X1′ of the gas has here a curved shape since the gastravels through the curved flow-shaping section 113.

The second flow-shaping section 114 has sidewalls 117 and a constrictionregion 118 in which its height varies along an axis perpendicular to thegas propagation axis X1X1′, with a height near its center less than theheight near the sidewalls 117. In one embodiment, the sidewalls 117 arediverging according to an angle A2 and the second flow-shaping section114 corresponds to the second expansion region previously described,which includes the constriction region. In another embodiment, thesecond flow-shaping section 114 has parallel sidewalls 117 and forms aconstrictor device for the gas wave expelled by the first flow-shapingsection 113, without further lateral expansion of the gas.

It will be noted that various other embodiments and applications of aninjector according to the invention may be provided by the skilledperson. The constriction region may have different shapes and may forexample be obtained by variations of both bottom and top inner surfacesof the flow-shaping section. The constriction region may also beimplemented using a diffuser plate of the type described later inconnection with FIGS. 12A, 12B, comprising openings being sized and/orspaced from each other so that the center of the gas wave is slowed downwith respect to its sides in the constriction region.

In addition, despite that it has indicated above that the system TFS2 isdesigned for Atomic Layer Deposition wherein gas waves (or gas pulses)are injected into the processing chamber, embodiments of an injectoraccording to the invention may also be used for other methods of andsystems for depositing materials, such as Chemical Vapor Deposition CVD,Physical Vapor Deposition PVD, Molecular Beam Epitaxy MBE,plasma-enhanced chemical vapor deposition PECVD, and in general anymethod wherein a gas travels through an injector and into a processingchamber.

Also, the use of the term “substrate” in the present description shouldbe taken to mean any type of material upon the surface of which achemical reaction may take place in order that thin film layers may beformed. These substrates may be of semiconductor material, plastic,metal, glass, optoelectronic devices, flat panel displays, liquidcrystal displays, etc. and may be of diverse sizes, shapes, and formats.

Equally, embodiments of an injector according to the invention may alsobe used to expand a continuous gas flow instead of shaping a gas wave.Such embodiments may help to improve a thin film deposition process byhomogenizing the quantity of reactant deposited by the gas over theentire processed surface, and consequently make the thickness of thedeposited thin film more uniform.

Also, a thin film deposition system according to the invention maycomprise a single injector instead of an injector assembly comprisingtwo or more injectors according to the invention.

In addition, embodiments of an injector according to the invention arenot only destined to make the gas velocity profile uniform at the outletof the injector. In other applications, it may be on the contrarydesired that the velocity of the gas near the sidewalls is differentthan the velocity of the gas the center of the outlet, and in particularit may be desired to obtain a greater velocity of the gas near thesidewalls relative to the velocity near the center of the outlet, aswill now be seen in connection with the description of a second aspectof the invention.

As indicated above, the application of thin film deposition techniquesfor processing substrates (500 mm or more) larger than conventionalsemiconductor substrates, requires that the dimensions of the injectors,the processing chamber, the exhaust device, etc. be modifiedaccordingly. In such applications, it may be desired to provide a thinfilm deposition system adapted for large substrates.

In such case, it may be desired to have injectors with a large outlet.However, it has been found that it becomes more and more difficult tocontrol the gas velocity profile at the outlet of an injector as itbecomes wider, i.e. as the ratio between the width of the outlet and thewidth of the inlet increases. Augmenting the width of the outlet whilekeeping the gas velocity constant at the outlet requires that theinjector have a greater length, that the amount of gas required totravel through the injector is increased, and that the gas is injectedwith a higher velocity at the inlet. All these constraints cause ahigher risk of turbulence and recirculation of the gas upon inlet, lessefficiency of the deposition process (longer time required between theinjection of successive gas pulses), non-uniform gas distribution,deposition anomalies, and uneven thin films.

FIG. 10 is a schematic top cross-sectional view of a thin filmdeposition system TFS3 according to a second aspect of the invention.The system TFS3 comprises a modular injector 100, a large processingchamber 42 shaped to receive a large substrate 52, and an adaptedexhaust device 62.

The modular injector 100 comprises at least two adjacent injectors, heretwo conventional injectors 1, l′ as described in relation with FIG. 1A.Both injectors are arranged side by side, with lateral walls 91 incontact with each other or separated by a joint. Each injector has aninlet coupled to a gas tube 71, 71′, to receive a gas wave (or a gasflow, depending on the process to be implemented), a flow-shapingsection 2, 2′ having left and right sidewalls that diverge according toa divergence angle, and an outlet for expelling the gas.

The modular injector 100 forms an equivalent large injector having anequivalent large outlet which includes the outlets of the adjacentinjectors 1, l′ and expanding the gas over the equivalent large outlet.The modular injector expels a resulting gas wave formed by thejuxtaposition of the gas waves expelled by each injectors, that travelsthrough the processing chamber 42 along a propagation axis X2X2′.

Such a parallel arrangement of injectors 1, l′ avoids the drawbacksindicated above, in particular it does not require an increase of thelength of the injectors and an increase of the velocity of the gasinjected at the inlet, thereby minimizing associated problems such asrecirculation and turbulence. In practice, the choice of the number ofinjectors to use depends on the size of the substrate to be processed.Furthermore, development costs are reduced due to optimization of asingle injector that is then used as many times as desired to obtain amodular injector.

In order to ensure the generation of a uniform gas wave upon the outletof the two or more injectors that form a modular injector, differentfactors must be taken into consideration, such as proper alignment ofthe injectors and synchronization of the gas waves for all injectors(timing and flow rate). These factors can be relatively easilyaddressed. The alignment of the two or more injectors with each otherand with the processing chamber can be achieved by any known means, suchas alignment pins, rails, etc.

Synchronization of the gas waves can be achieved with a fairly highaccuracy by the precise control of gas sources, valves, and gas tubeswith nominally identical components for each modular injector. Forexample, electrically-actuated pneumatic valves allow for the injectionof gas pulses with durations as low as 20 ms and with synchronization ofseveral values with an accuracy better than 1 ms.

Another factor that may be taken in consideration is uniformity of thepulse over the equivalent large outlet of the modular injector. Now,referring to FIG. 10, it can be noted that a connection area 90 existsbetween the injectors, where the two injectors 1, l′ are joined. Theconnection area 90 is defined as a distance between the adjacentsidewalls 91 of the outlets of the injectors 1, V. More generallyspeaking, in a modular injector comprising more than two injectors, itis defined as the distance between adjacent outlet sidewalls of any twoadjacent injectors. Such a connection area is generally equal to twotimes the thickness of the walls of the injectors that are side by side.It may also include the thickness of any alignment or fastening meansthat may be arranged between two adjacent injectors.

It can be further noted that since no gas is expelled in the connectionarea, the gas velocity profile may present a non-uniform region 92opposite the connection area, along the propagation axis X2X2′ of theresulting gas wave. This phenomenon is worsened by the fact that theconventional injectors 1, l′ intrinsically present a non-uniformdistribution of the gas upon their outlets, with a lower gas velocitynear their sidewalls.

FIG. 11 is a schematic top cross-sectional view of a thin filmdeposition system TFS4 according to the second aspect of the invention,which provides a better uniformity of the gas velocity profile in theprocessing chamber. The system TFS4 is generally similar to system TFS3and also comprises a modular injector 101 with two injectors 1, l′, theprocessing chamber 42 and the exhaust device 62.

Each injector 1, l′ further comprises a diffuser plate 95, 95′, arrangedat its outlet. Each diffuser plate 95, 95′ is designed to increase theflow rate of the gas near the adjacent sidewalls of the injectors, tocompensate for the lack of gas expulsion in the connection area 90. As aresult, the large gas wave in the processing chamber shows a uniformvelocity profile at a certain distance from the outlet of the modularinjector 101, in a region 93 opposite the connection area.

FIG. 12A shows an example embodiment 95-1 of the diffuser plates 95,95′. Diffuser plate 95-1 comprises openings 96-1 with constant diameterand non constant distribution, so that there are more openings near theedges of plate 95-1 corresponding to the sidewalls of the injector, thannear the center of the plate. For example, the center-to-center distancebetween the openings varies from a minimum value S1 at the edges of theplate to a maximum value S2 at the center of the plate, where theopening are wider spaced.

FIG. 12B shows another example embodiment 95-2 of the diffuser plates95, 95′. Diffuser plate 95-2 comprises openings 96-2 with varyingdiameters and constant center-to-center distance, arranged so that thereare larger openings near the edges of plate 95-2 corresponding to thesidewalls of the injector, than in the center of the plate. For example,the diameter of the openings varies from a minimum value D1 near thecenter of the plate to a maximum value D2 at the edges of the plate.

A third embodiment of the plates 95, 95′, not shown, may comprisevariable center-to-center distance between the openings and variableopening diameters.

It is to be noted in FIGS. 12A, 12B that the openings of the diffuserplates 95-1, 95-2 have similar arrangement at right and left edgesdiffuser plates. This symmetrical arrangement increases the gas flowrate at each side of the outlets of injectors 1, 1′, to compensate forboth the absence of gas expulsion in the connection area 90 and thepresence of non-adjacent lateral walls 91′ at each side of the modularinjector. Therefore, the equivalent width of the outlet of the modularinjector can be considered as the sum of the widths of each outlet, plusthe sum of the thicknesses of the adjacent walls 91, plus the sum of thethicknesses of the non adjacent lateral walls 91′. In other embodiments,the openings of the diffuser plates 95-1, 95-2 may have nonsymmetricalarrangement at right and left edges diffuser plates.

FIG. 13 is a schematic top cross-sectional view of another embodimentTFS5 of a thin film deposition system according to the second aspect ofthe invention, which also provides a better uniformity of the gasvelocity profile in the processing chamber and compensates the presenceof the connection area 90.

The system TFS5 comprises a modular injector 102, the processing chamber42, the exhaust device 62, and the connection area 90. The modularinjector 102 comprises two adjacently arranged injectors 211, 211′ thatare designed according to the first aspect of the invention, eachinjector comprising an expansion region 25 and a constriction region 24.Each injector 211 or 211′ differs from injector 11 or 11′ previouslydescribed in that the expansion and constriction regions are configuredso that, at the outlet of the injector, the velocity of the gas near thesidewalls is greater than the velocity of the gas near the center of theoutlet, which also means that the flow rate of the gas near thesidewalls is greater than the flow rate of the gas near the center ofthe outlet.

In a nonsymmetrical embodiment of the injector 211, 211′, the expansionand constriction regions may be configured so that velocity of the gasis only greater near the sidewall which is adjacent to a sidewall of theother injector.

FIG. 14 shows the profile C5 of the gas velocity V at the outlet of theinjectors 211, 211′ of FIG. 13. The gas velocity is measured along anaxis Y5Y5′ which is perpendicular to the propagation axis of the largegas wave, and is located at a certain distance d5 from the outlet of themodular injector, for example at 5 mm from the modular injector. Thevelocity profile C5 is substantially constant and presents a variationof less than 10% between maximum and minimum velocities over at least90% of the width of the equivalent large outlet of the modular injector.

A modular injector is susceptible of various other embodiments andapplications. In one embodiment, several modular injectors, eachcomprising N parallel injectors, are arranged in a thin film depositionsystem comprising a conveyer belt upon which substrates are arranged(in-line system) or a continuous roll of a flexible substrate(roll-to-roll system). Each time a substrate or a part of the continuoussubstrate is in front of the outlet of a modular injector, one or morepulses of one or more gases are expelled onto the substrate. Such asystem allows for the continuous deposition of thin films on a series ofsubstrates, resulting in higher throughput.

FIGS. 15A, 15B, 15C show different arrangements for supplying a modularinjector 100, 101, 102 with gas. As it is desired to obtain a uniformgas profile at the outlet of the modular injector, it is important toprecisely synchronize the injection of the gas into each injector, inparticular when a pulsed process is implemented. In FIG. 15A, a singlegas source 81 is linked to the injectors 1, l′ or 211, 211′ through asingle valve 85. In FIG. 15B, a single gas source 81 is linked to theinjectors 1, l′ through two valves 85, 85′, one per injector. In FIG.15C, one first gas source 81 is linked to the first injector 1 or 211through one first valve 85. One second gas source 81′ providing the samegas as the first gas source 81, is linked to the second injector 1′ or211′ through one second valve 85′. It will be clear to the skilledperson that these arrangements are applicable to modular injectorscomprising N parallel injectors; in particular, N parallel injectors maybe driven by a single valve.

Furthermore, despite that the present invention has been described abovein connection with applications relating to thin film depositiontechniques, it will be clear to the skilled person that embodiments ofan injector or embodiments of a modular injector according to theinvention may be used for other purposes, in different applicationswhere it is required to inject a gas in a processing chamber, such asetching, diffusion, and the like.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

I claim:
 1. A modular injector for injecting a gas into a processingchamber, the modular injector comprising at least two adjacentinjectors, each injector comprising: an inlet for receiving a gas waveor a gas flow; a flow shaping section having left and right sidewallsthat diverge according to a divergence angle relative to a propagationaxis of the gas for expanding the gas in a direction perpendicular tothe propagation axis; an outlet for expelling the gas; and a connectionarea extending between adjacent sidewalls of the at least two adjacentinjectors, wherein the modular injector forms a substantially equivalentlarge injector having an equivalent large outlet including the outletsof the at least two adjacent injectors and expands the gas over theequivalent large outlet, and wherein the flow-shaping section of eachinjector comprises at least a constriction region where a height of theflow-shaping section varies along an axis perpendicular to thepropagation axis and presents a first height near a center of theflow-shaping section and a second height near a sidewall close to theconnection area, the first height being less than the second height toslow down a velocity of the gas near the center of the gas relative tothe velocity of the gas near the sidewall close to the connection area,such that each injector expels the gas in a vicinity of the connectionarea with a greater flow rate than near a center of its outlet in orderto compensate for a lack of gas expulsion in the connection area.
 2. Themodular injector according to claim 1, wherein the injectors areconfigured so that the gas expelled by the modular injector through theequivalent large outlet has a velocity profile showing a variation ofless than 10% between maximum and minimum velocities over at least 90%of the width of the equivalent large outlet, at a certain distance fromthe outlet and the connection area.
 3. The modular injector according toclaim 1, wherein the flow-shaping section of each injector has a firstexpansion region where the sidewalls diverge according to a firstdivergence angle, and a second expansion region including theconstriction region, where the sidewalls diverge according to a seconddivergence angle, smaller than the first divergence angle, to acceleratea velocity of the gas near the sidewalls close to the connection arearelative to a velocity of the gas near the center of the flow-shapingsection.
 4. The modular injector according to claim 3, wherein the firstdivergence angle varies and presents a largest value at an end of thefirst expansion region, and the second divergence angle is constant andsmaller than a largest value of the first divergence angle.
 5. Themodular injector according to claim 1, having a curved flow shapingsection.
 6. A system comprising: a processing chamber, at least onemodular injector according to claim 1, an equivalent large outlet ofwhich opens into the processing chamber, and at least one gas sourcecoupled to inputs of the injectors of the modular injector.
 7. Thesystem according to claim 6, comprising at least two superposed modularinjectors arranged so that each injector of each modular injector has acommon outlet with one injector of another modular injector.
 8. A methodfor injecting a gas wave or a gas flow into a processing chamber, themethod comprising: expanding the gas in a direction perpendicular to apropagation axis of the gas; then injecting the gas into the processingchamber using a modular injector comprising at least two adjacentinjectors, a connection area extending between adjacent sidewalls of theat least two adjacent injectors, each injector comprising an inlet forreceiving a gas, a flow-shaping section having left and right sidewallswhich diverge according to a divergence angle relative to a propagationaxis of the gas, for expanding the gas in a direction perpendicular tothe propagation axis, and an outlet for expelling the gas, the modularinjector forming an equivalent large injector having an equivalent largeoutlet including the outlets of the at least two adjacent injectors andexpanding the gas over the equivalent large outlet; and providing, inthe flow-shaping section of each injector, at least a constrictionregion where a height of the flow-shaping section varies along an axisperpendicular to the propagation axis and presents a first height near acenter of the flow-shaping section and a second height near a sidewallclose to the connection area, the first height being less than thesecond height to slow down a velocity of the gas near the center of thegas relative to the velocity of the gas near the sidewall close to theconnection area, such that each injector expels the gas in a vicinity ofthe connection area with a greater flow rate than near a center of itsoutlet in order to compensate for a lack of gas expulsion in theconnection area.
 9. The method according to claim 8, comprisingconfiguring each injector so that the gas expelled by the modularinjector has a velocity profile showing a variation of less than 10%between maximum and minimum velocities over at least 90% of a width ofthe equivalent large outlet.
 10. The method according to claim 8,comprising providing in the flow-shaping section of each injector, afirst expansion region where the sidewalls diverge according to a firstdivergence angle, and a second expansion region including theconstriction region, where the sidewalls diverge according to a seconddivergence angle, smaller than the first divergence angle, to acceleratea velocity of the gas near the sidewalls close to the connection arearelative to a velocity of the gas near the center of the flow-shapingsection.
 11. The method according to claim 10, comprising providing afirst divergence angle which varies and presents a largest value at anend of the first expansion region, and a second divergence angle whichis constant and smaller than a largest value of the first divergenceangle.
 12. A method of manufacturing a device comprising a substrate anda thin film, the method comprising: a step of depositing the thin filmonto the substrate by: arranging the substrate in a processing chamber;injecting a gas wave or a gas flow into a processing chamber; expandingthe gas in a direction perpendicular to a propagation axis of the gas;then injecting the gas into the processing chamber using a modularinjector comprising at least two adjacent injectors, a connection areaextending between adjacent sidewalls of the at least two injectors, eachinjector comprising an inlet for receiving a gas, a flow-shaping sectionhaving left and right sidewalls which diverge according to a divergenceangle relative to a propagation axis of the gas, for expanding the gasin a direction perpendicular to the propagation axis, and an outlet forexpelling the gas, the modular injector forming an equivalent largeinjector having an equivalent large outlet including the outlets of theadjacent injectors and expanding the gas over the equivalent largeoutlet; and providing, in the flow-shaping section of each injector, atleast a constriction region where a height of the flow-shaping sectionvaries along an axis perpendicular to the propagation axis and presentsa first height near a center of the flow-shaping section and a secondheight near a sidewall close to the connection area, the first heightbeing less than the second height to slow down a velocity of the gasnear the center of the gas relative to the velocity of the gas near thesidewall close to the connection area, such that each injector expelsthe gas in a vicinity of the connection area with a greater flow ratethan near a center of its outlet in order to compensate for a lack ofgas expulsion in the connection area.
 13. The method according to claim12, comprising providing in the flow-shaping section of each injector, afirst expansion region where the sidewalls diverge according to a firstdivergence angle, and a second expansion region including theconstriction region, where the sidewalls diverge according to a seconddivergence angle, smaller than the first divergence angle, to acceleratea velocity of the gas near the sidewalls close to the connection arearelative to a velocity of the gas near the center of the flow-shapingsection.
 14. The method according to claim 13, comprising providing afirst divergence angle which varies and presents a largest value at anend of the first expansion region, and a second divergence angle whichis constant and smaller than a largest value of the first divergenceangle.